1. Field of the Invention
The present invention relates to an electron emission device and particularly to a porous semiconductor electron emission device.
2. Description of the Related Art
In field electron emission display apparatuses, an FED (Field Emission Display) is known as a planar emission display device equipped with an array of cold-cathode electron emission source which does not require cathode heating. There also exists attentions to planar electron sources such as an electron emission device with a metal-insulator-metal (MIM) structure and to an electron emission device using a porous semiconductor such as silicon (Si) with a homogeneous porosity.
As shown in FIG. 1, the electron emission device using a porous semiconductor layer comprises a porous semiconductor layer 13 and a thin-film metal electrode 15 which are formed in turn on a silicon layer 12 provided with an ohmic electrode 11 at the back side.
The porous semiconductor electron emission device can be regarded as a diode of which the thin-film metal electrode 15 at its surface is connected to a positive potential Vps and the back i.e., ohmic electrode 11 is connected to a ground potential. When the voltage Vps is applied between the ohmic electrode 11 and the thin-film metal electrode 15 to supply electrons into the semiconductor layer 12 of semiconductor Si, a diode current Ips flows. Since the porous semiconductor layer 13 has a high resistance, most of the applied electric field is applied to the porous semiconductor layer 13. The electrons travel inside the porous semiconductor layer 13 toward the thin-film metal electrode 15. Some of the electrons that reach near the thin-film metal electrode 15 tunnel through the thin-film metal electrode 15, due to the strong electric field, to be discharged out into the vacuum space 4. The electrons e (emission current Iem) discharged from the thin-film metal electrode 15 by the tunnel effect are accelerated by a high voltage Vc, which is applied to an opposing collector electrode (transparent electrode) 2, and is collected at the collector electrode 2. If a fluorescent substance is coated on the collector electrode 2, corresponding visible light is emitted.
To fabricate a color display panel using the porous semiconductor electron emission device, the collector electrodes 2 capturing emitted electrons are grouped per three for R, G and B color signals corresponding to Red, Green and Blue emitting portions. Thus, fluorescent material layers 3R, 3G and 3B corresponding to R, G and B emissions are formed on the corresponding collector electrodes 2 onto which the electrons caused by the tunnel effect are impinged after the travel in the vacuum space 4.
However, there are problems as follows:
(1) When the diode current Ips is in an over-current, the electron emission efficiency .eta. (emission current Iem/diode current Ips) decreases; PA1 (2) The surface of the porous semiconductor layer is considerably rough, and its contact to thin-film metal electrode prepared later becomes defective, so that the electron emission becomes unstable in the element. PA1 (3) Since the thermal conductivity of the porous semiconductor layer of Si is low in comparison with that of the original silicon, heat destruction of the element tends to occur. PA1 a semiconductor layer for supplying electrons; PA1 a porous semiconductor layer formed on the semiconductor layer; and PA1 a thin-film metal electrode which is formed on the porous semiconductor layer and faces a vacuum space; wherein the porous semiconductor layer has at least two or more of porosity-changed layers which have porosities which are different from each other in the thickness direction, whereby the electron emission device emits electrons when an electric field is applied between the semiconductor layer and the thin-film metal electrode. PA1 a semiconductor layer for supplying electrons; PA1 a porous semiconductor layer formed on the semiconductor layer; PA1 an insulator layer formed on the porous semiconductor layer and made of a material selected from silicon oxide or silicon nitride; and PA1 a thin-film metal electrode which is formed on the insulator layer and faces a vacuum space; whereby the electron emission device emits electrons when an electric field is applied between the semiconductor layer and the thin-film metal electrode. PA1 a semiconductor layer for supplying electrons; PA1 a porous semiconductor layer formed on the semiconductor layer; and PA1 a thin-film metal electrode which is formed on the porous semiconductor layer and faces a vacuum space; wherein skeletons of the porous semiconductor layer are oxidized or nitrided, whereby the electron emission device emits electrons when an electric field is applied between the semiconductor layer and the thin-film metal electrode.
On the other hand, the porous silicon layer is formed by anodization to a silicon film in a mixed solution of hydroflouric acid and ethyl alcohol.
By heating the porous silicon layer at a high temperature in a vacuum after the anodization thereof, hydrogen terminuses existing adjacent to the surface of the porous silicon layer are removed, and then dangling bonds are terminated with atoms of oxygen or nitrogen through some processes such as heating or plasma-processing in oxygen gas or nitrogen gas, so that the porous silicon layer becomes stable.
However, even by use of the above termination method, there is a difficulty to control the thickness of the portion terminated with oxygen and nitrogen in the porous silicon layer. Optimization of terminus processing conditions is because it is hard. Therefore, since the porous silicon portion terminated with oxygen and nitrogen is an important condition for electroluminescence (EL) and photoluminescence (PL), any uniform stable EL and PL light-emissions are not yet obtained from the porous semiconductor electron emission device.